1. Field of the Invention
This invention relates to a biodegradable plastic material having improved durability and, also, a biodegradable plastic molding using same.
2. Description of the Related Art
Today, plastics are prevalent in all aspects of daily life and industry, and annual production of plastics all over the world has reached approximately one hundred million tons. The greater part thereof is disposed of after use, and this has been recognized as one of the factors causing disruption in the global environment. At present, recycling of plastics, as well as using biodegradable polymers, are attracting attention as measures for solving this problem.
With respect to recycling plastics, although the Law for Recycling of Specified Kinds of Home Appliances was enacted in Japan in April 2001 in order to recycle used electric appliances, the recycling of used electric appliances has been, at best, inadequate, with the exception of four kinds of large-sized electric appliances including televisions, refrigerators, air conditioners and washing machines. Instead, most electric appliances are disposed of as incombustible refuse. Also, when sold in large quantities, even small-sized electric appliances end up as huge amounts of waste material and present problems in terms of environmental preservation.
Shredding waste material is a popular processing method employed today. However, since shredding merely decreases the volume of waste material, and since the waste material, if buried underground, remains unchanged over several ten years or several hundred years, shredding does not bring any basic solution to this environmental problem. Even if the shredder dust were to be recycled, because it has been finely reduced, valuable material such as copper would become mixed with other less valuable material, for example, and its purity would be compromised, thereby lowering collection efficiency.
On the other hand, it could be inferred that using biodegradable polymers has the following two advantages. First, by manufacturing the casing and structure of an electronic appliance, both of which occupy the bulk of a product's volume, with biodegradable material, and by employing an easily separable structure such as, for example, screws or fitting (male-female) structures to assemble these parts with non-biodegradable parts, such as electronic components and substrates, parts to be recycled and parts which can be disposed of can be disassembled and processed individually with relative ease. Thus, improved collection efficiency could be expected.
Second, biodegradable material may be used for the external surfaces of the casings of products such as radios, microphones, portable televisions, keyboards, Walkmans, portable telephones, radio cassette recorders and earphones, for instance. By thus employing biodegradable material for portions of devices which frequently contact a user's body as described above, electrical appliances, which are safer than those employing synthetic resins for comparable portions, may be provided.
However, not every kind of biodegradable polymer is suitable for use in the manner described above, and in order to be suitable for use as a material for the casing or structure of an electrical appliance, certain physical properties are required. The present inventors found that it is at least required that no degradation of the physical properties take place even if held for 48 hours in an atmosphere of 80° C. in temperature and 80% humidity.
Biodegradable polymers are organic materials, which are degradable and assimilated into the environment by the action of nature or microorganisms, and which have been developed as ideal materials that meet environmental needs. Examples of such biodegradable polymers described above may include: polysaccharide derivatives such as, for instance, cellulose, starch, dextran, chitin and the like; peptides such as, for example, collagen, casein, fibrin, gelatin and the like; polyamino acids; polyvinyl alcohol; polyamides such as, for example, 4-nylon and 2-nylon/6-nylon copolymer; aliphatic polyesters; and so forth.
Aliphatic polyester resin, which is a typical example of a biodegradable polymer, generally has a low melting point and is inadequate in terms of its physical properties (particularly, heat resistance and shock resistance) to be fit for use in commercially viable moldings. Considerations such as adding inorganic fillers or crystal nucleic agents for improving the crystallization rate or blending with biodegradable resins showing rubber-like properties and having low glass transition points have been made. Several patent applications for such moldings using plastics described above have already been filed (Japanese Unexamined Patent Application Publications No. 3-290461, No. 4-146952, No. 4-325526 and the like). These moldings are used as films or packaging materials, and hence do not specifically require durability.
On the other hand, in applying biodegradable aliphatic resin to the casings of electrical appliances and electronic devices and the like, long-term reliability, in other words, durability under conditions of constant temperature and humidity, as well as heat resistance, are required. Although product life cycles vary between electric appliances and electronic devices, in small-sized audio products, it is necessary that their physical properties be maintained for 3 to 7 years under, for example, conditions of 30° C. and 80% relative humidity. Considering the fact that electric appliances and electronic devices are operated under widely varying temperatures and humidity conditions, since existing biodegradable polyesters have issues in terms of long-term reliability, as described above, they could not be used for casings of electric appliances, electronic devices and the like. Currently, biodegradable polymers, mainly aliphatic polyester resins, are beginning to be used as materials for use in agriculture, forestry and fishery (films, planters, fishing lines, fishing nets and the like), and materials for use in civil engineering (water-retentive sheets, plant nets, sandbags and the like), as well as in the fields of packaging and containerization (for those which are hard to recycle due to adhesion of soil and food thereto).
As described above, it is at least required of biodegradable polyester resins that no degradation of their physical properties take place for at least 48 hours under conditions of constant temperature and humidity (for instance, at 80° C. and 80% relative humidity) if they are to be used in casings of electric appliances, electronic devices and the like. Moldings of existing biodegradable polyesters, even in the case of polylactic acid, for example, which is most resistant to heat, when subjected to aging tests over 48 hours at a temperature of 80° C. and a relative humidity of 80%, a 60% decrease in molecular weight by the action of hydrolysis takes place (Refer to Comparative example 1 below), and thus using them as materials for casings of household electric appliances is difficult. As one of the factors accounting for such a degradation in physical properties, that is, the occurrence of hydrolysis, it is known, for example, in the case of polyester, that a carboxyl group at a polymer chain terminal catalytically causes hydrolysis of ester bonds in molecular chains.